What causes trembling/shuddering in arms and legs ?

A talk amongst our Stroke Support Group bought up some questions/conversation about trembling. To give context, one of our members had regained her drivers license that morning. She talked of her legs trembling, not just nerves during her test. When I asked did she know why it happened she replied know, then I asked would she like to know. To which she replied yes so I said I would try to find out more about this before our next meeting. She was still trembling at the meeting and appeared quite uncomfortable about it. I then recalled my own experience of when I regained my license of how my left (stroke affected) leg had shuddered during the test, which was very off putting. I was driving a manual as well so extra work for my left leg. I know I showed dismay and uncertainty to the instructor at the time and he replied to me that this "tremble/shudder" was normal. This didn't give me any answers so I just focused on driving and getting through the test - which I passed!! Is their anything which I can pass on to our group to give them some clarity about this and what people could possibly do about it?